The present invention relates to a method of, and apparatus for, heating an asphalt-paved road surface.
As used herein, the term xe2x80x9casphaltxe2x80x9d is meant to include macadam and tarmac. As is known in the art, asphalt-paved road surfaces typically comprise a concrete mixture of asphalt cement (typically a black, sticky, petrochemical binder) and an aggregate comprising appropriately sized stones, gravel, and/or sand. The asphalt concrete mixture is usually laid, compressed, and smoothed to provide an asphalt-paved road surface.
Conventionally, in repair work of roads paved with asphalt concrete, the road surfaces are, prior to the repair work, softened through heating thereof by road surface heating apparatuses, each typically mounted on a vehicle. It is necessary to soften old asphalt so that a satisfactory bond is formed between the old asphalt and subsequently applied new asphalt. Softening is also required where portions of the old asphalt are to be recycled into the new pavement. Such recycling generally operates on the premise of (1) heating the paved surface to soften an exposed layer of asphalt using direct flames, infrared burners, radiant heaters, hot air blowers, or other heating means; (2) mechanically breaking up the heated surface, typically using devices such as rotating, toothed grinders; screw auger/mills; and rake-like scarifiers; (3) applying fresh asphalt and/or asphalt rejuvenant to the heated, broken asphalt; (4) distributing the mixture over the road surface; and (5) compacting or pressing the distributed mixture to provide a recycled asphalt-paved surface.
U.S. Pat. No. 5,653,552 issued to Wiley and U.S. Pat. No. 5,791,814 issued to Wiley et al. discuss in great detail a number of problems in respect of heating, softening, and rupturing old asphalt. For example, asphalt concrete (especially the asphalt cement within it) is susceptible to damage from heat, usually when the asphalt mixture is heated to a temperature above approximately 320xc2x0 F. (160xc2x0 C.), although old asphalt pavement can usually be heated to a temperature approaching the flash point of asphalt (about 210xc2x0 C. or 410xc2x0 F.) since the surface oil on the pavement has typically been worn off, washed away, or severely oxidized. Another problem in this regard is the increasing difficulty of heating asphalt concrete as the depth of the layer being heated increases. A further problem results from excess heating and/or smoking of the asphalt surface which can lead to a negative impact on the environment.
To this end, U.S. Pat. No. 5,653,522 and U.S. Pat. No. 5,791,814 discuss in detail numerous attempts in the prior art to deal with the inherent difficulty of adequately and uniformly heating an asphalt surface in an efficient manner while minimizing or eliminating burning and smoking of the asphalt surface. Much of this effort involved utilizing relatively complicated means to distribute heat through the asphalt surface after rupturing thereof, often requiring further heating of the ruptured asphalt surface to facilitate heat distribution therethrough. These complicated processing means were typically cumbersome and large, yet were necessary due to the inability to preheat the unruptured asphalt surface adequately without overheating thereof.
U.S. Pat. No. 4,561,800 issued to Hatakenaka et al. and U.S. Pat. No. 4,559,922 issued to Crupi et al. teach the use of blowing hot air (or other gases) to heat and soften a road surface, and then recirculating and reheating those hot gases. In particular, each patent teaches an apparatus having an enclosure with a bottom peripheral wall positionable in engagement with a road surface, a heater, a duct for conveying hot gases from the heater to the enclosure and to the road surface under the enclosure, a duct for recycling gases after their contact with the road surface back to the heater for reheating, and a fan for circulating the hot gases through the ducts. According to these patents, using blowing hot gases resolves problems with direct flame and radiant heating means for heating a road surface, namely that the combination of the heat applied in accordance with those means and the oxygen in the atmosphere tended to oxidize the asphalt and drive off relative volatile components, thereby causing deterioration of the quality of the asphalt as well as releasing objectionable pollutants into the air. In contrast, by recirculating hot gases, minimal pollutants are released into the surrounding atmosphere and the hot gases can be controlled so as to have very little oxygen, thereby minimizing oxidation of the asphalt. Another problem with direct flame and infrared burners is that they result in non-uniform heating of the road surface, with the portions closely confronting the burners being overheated and burned, while other portions are underheated. By instead blowing hot gases uniformly across the road surface, Hatakenaka et al. and Crupi et al. provide a means of more uniformly heating a road surface.
Hatakenaka et al. goes one step further than Crupi et al., in that Hatakenaka et al. also teaches the use of a thermal control to maintain the hot gases in the duct leaving the heater at a constant, pre-determined temperature. However, this still would not prevent the possibility of overheating the road surface since Hatakenaka et al. does not disclose means by which the temperature of the road surface is taken into account and by which the constant, pre-determined temperature of the hot gases can be automatically reduced as the road surface approaches the flash point of the asphalt.
All of the prior art continues to exhibit a persistent problem in controlling the heat source in such a manner that the heat being produced is relative to only what is required to efficiently heat the asphalt material without causing damage. In addition, the prior art processes and apparatuses generally require that the material be heated only from the top of the road surface or in combination with a secondary heating step that applies heat to ruptured material. While rupturing the material and exposing more free oil creates a black surface that more readily absorbs infrared waves, it has been discovered that heating this exposed oil in loosened material greatly increases the amount of deterioration in the asphalt due to heat. Accordingly, it is preferable to heat the asphalt surface sufficiently prior to rupturing.
The present invention provides an improved method and apparatus for heating an asphalt-paved road surface to soften it prior to initiating repair work. This method and apparatus involves forcing gases heated by a heater against that road surface and then returning those gases to the heater for reheating and recirculation, wherein the temperature of the returning gases is measured by a temperature sensor, and the heater is automatically adjusted so that the temperature of the gases being directed against the road surface is automatically decreased as the temperature of the returning gases increases. This prevents damage to the asphalt and premature rupturing of the road surface.
In particular, the method according to the present invention comprises: (1) heating gases adjacent the heater; (2) forcing gases heated by the heater into contact with the road surface; (3) collecting the gases after they have been forced against the road surface and returning them to the heater; (4) measuring the temperature of the gases as they are returned from the road surface to the heater; and (5) adjusting the heater so that the temperature to which gases are heated by the heater decreases as the temperature of the returned gases increases. To this end, a road surface heating apparatus according to the present invention comprises: (1) a heater for heating gases; (2) at least one manifold for directing heated gases from the heater against a road surface; (3) an enclosure for trapping gases exiting that manifold against the road surface; (4) a fan for returning gases which have been directed against the road surface back to the heater for reheating and recirculating; and (5) a temperature sensor for measuring the temperature of gases being returned to the heater, wherein the heater of this heating apparatus provides heated gases at a temperature that decreases as the temperature measured by the temperature sensor increases. This apparatus could further comprise a venturi valve through which the fan directs returning gases at high velocity to the heater, creating a low pressure area sufficient to allow the heater to be no more than a natural aspirated burner introduced into the air stream to reheat the gases and combust any fumes collected. The temperature sensor can be a simple thermocouple.
The efficiency of this method and apparatus can be improved even further by doing the following: once the road surface has been heated for a period of time according to the method and apparatus described above, grooves can be pressed into the heat-softened road surface (for example, by a flanged reforming drum) without rupturing it. This prepares the road surface for further and deeper and more effective heating in accordance with the method and apparatus described above.
After completion of heating and softening of a road surface in accordance with the method and apparatus described above, the road surface can then be ruptured (for example, by a rupturing drum) and the ruptured material can then be reused in new pavement for the road surface.